1. Field of the Invention
This invention relates in general to a bearing structure for a spindle motor, and more particularly to a fluid bearing seal and support structure for use in a spindle motor.
2. Description of Related Art
Disk drives are computer mass storage devices from which data may be read and/or to which such data may be written. In general, they comprise one or more randomly accessible rotating storage media, or disks, on which data is encoded by various means. In magnetic disk drives, data is encoded as bits of information including magnetic field reversals grouped in tracks on the magnetically-hard surface of the rotating disks. The disks are stacked in a generally parallel and spaced-apart relationship and affixed at their inner diameter ("ID") to a common hub which is rotationally coupled to a stationary spindle shaft by a pair of bearings, typically ball bearings.
With the growing trend toward even lower height form factor disk drives, the length of the spindle shaft and spacing between the upper and lower bearings becomes a significant consideration in meeting specific drive height constraints. As drive height is decreased, a proportionately shorter spindle must be accommodated within the decreased height constraints with a concomitantly shorter spacing available between the upper and lower bearings supporting the hub on the spindle.
Rotary spindle motors having fluid bearings for supporting the rotary member rather than traditional ball bearings typically include a shaft having at least one axial thrust plate and a hub, which may be a rotary hub, having a sleeve portion generally enclosing the shaft and thrust plate, thus forming a journal bearing with bearing fluid disposed therein. The bearing fluid will form capillary seals at one or more ends of the shaft that are exposed to ambient air pressure.
The problem with such constructions is that under certain conditions the capillary seal may break down and fluid may leak from the bearing. Disruption of the seal may be caused by shock or vibration. Under certain conditions the rotating portion of the bearing may be displaced along the axis of the shaft. In the normal course of events, lubricant flows around the end of the thrust plate from the side with decreasing clearance to the side with increasing clearance. If, however, because of sudden shock or vibration, the lubricant flow around the thrust plate is impeded, fluid will be pushed toward one end of the shaft or the other, possibly breaking down the surface tension which holds the seal in place.
Leakage may also occur when there are inaccuracies in the fabrication of the patterned grooves used by the thrust plate's upper and lower surfaces to maintain a desired net pressure gradient. The object of the grooves is to create a high pressure region in the middle of each thrust plate surface and to create ambient pressure zones at the inner diameter of the thrust plate, adjacent the shaft, and at the outer diameter in the gap between the readily outermost edge of the thrust plate and the sleeve. This type of pressure distribution ordinarily results in no displacement of bearing fluid, that is, the static pressures will equalize. However, fabrication inaccuracies do occur, as does tilt in the bearing, or any other physical phenomena, and these factors can alter the pressure balance in the bearing fluid resulting in flow across the bearing. The flow of bearing fluid can overcome the surface tension seal at either end of the bearing and cause the fluid to leak. The situation is particularly acute at the thrust plate end where pressure imbalances between the upper and lower surfaces of the thrust plate may create a net flow which is near the capillary seal at the upper surface of the thrust plate.
Nevertheless, prior axial bearing support structures have not simultaneously address preventing oil from leaking out, maintaining the bearing integrity under shock, reducing oil evaporation and minimizing distortion of the active bearing surface. Rather, existing designs have addressed only a fraction of the requirements, e.g., only evaporation and shock induced bearing separation or distortion and evaporation.
For example, U.S. Pat. No. 5,490,021, issued Feb. 6, 1996, to Johannes C. A. Muller et al., and assigned to U.S. Phillips Corporation, herein incorporated by reference, disclosed a dynamic groove bearing for a hard disk drive. The dynamic groove bearing includes a sleeve-like housing having a locking piece that includes a bearing disk portion which cooperates with an annular bearing disk provided on a shaft. A pressure member is adapted to compress an annular, elastically deformable sealing element to thereby seal the interface between the housing and the locking piece and to pretension the locking piece against a seat formed on the housing. However, Muller et al. does not address evaporation and shock induced bearing separation.
It can be seen that there is a need for an axial bearing support structure that prevents oil from leaking out, maintains the bearing integrity under shock, reduces oil evaporation and minimizes distortion of the active bearing surface.